A Microsoft application for managing email, calendars, contacts, and tasks across devices and platforms.
So, I had to park this, as I was travelling a lot for work, and barely using the PC that had this issue.
I have since returned, and still had the problem. However, I noticed on my account details under Excel that it had my wrong Office365 subscription (it was an old one that I had more than 2 years ago upgraded!).
I upgraded the sub, and so far since, it has all worked as supposed to.
So that has worked for me - might be worth checking (if you are on Office365), but I suspect this won't be the issues for many people, and that there is therefore still a problem somewhere that ideally MS needs to take a look at!